In a preferred application, the present invention relates to a device and a method for treating deafness that makes use of emitting ultrasound waves into the brain of a patient suffering from deafness or hearing difficulties.
In another preferred application, the present invention relates to a device and a method for treating vision that makes use of emitting ultrasound waves into the brain of a patient suffering from blindness or visual difficulties.
In the field of audition, deafness and hearing difficulties nowadays constitute a major medical problem insofar as these pathologies have considerable repercussions on the social lives of the patients concerned.
In general, the term “deafness” is used for patients suffering from a deep loss of hearing, generally a loss greater than 90 decibels (dB).
Three main categories of deafness are presently known.
Firstly there is so-called “conductive” deafness that involves pathologies of the middle ear.
There is also so-called “sensorineural” deafness that relates to diseases of the cochlea, situated in the inner ear or to malformations of the cochlea.
Finally, there is also so-called “neural” deafness which results from pathologies of the auditory nerve or the auditory cortices of the brain.
The physical and physiological mechanisms of hearing are particularly complex to understand.
The outer ear collects sounds and concentrates them inside the outer ear on the eardrum. The eardrum then transmits the perceived sounds by setting into vibration the ossicular chain contained in the cavity of the middle ear. The middle ear acts as a transmission amplifier with amplification being performed by the three small bones of the ossicular chain. The sound signal as amplified by the middle ear is then transmitted to the fluid-filled inner ear that is formed by the cochlea. The inner ear then transforms the signal into nervous excitation of the cochlear nerve. Each sound frequency corresponds to one or more specific activations of each of the fibers of the cochlear nerve.
The cochlear nerve is directed towards the eighth nucleus of the brain stem and the neurons relay and conduct the neural signal towards the primary auditory cortex known as Heschl's gyrus in Broadmann's area 41, and also to Broadmann's area 42 of the planar temporal gyrus (A. Carpentier et al., Neurochirurgie 2002; 48(2-3): pp. 80-86).
Thereafter, the neural signal is processed by secondary auditory areas (also known as “associative zones”) of the brain before reaching the language area (Broadmann's area 22, also known as Wernicke's area).
In all cases of hearing difficulties and deafness, various solutions now exist to enable patients to be fitted, in particular with cochlear implants or indeed with brain-stem implants. For example, more than 188,000 patients throughout the world are receiving cochlear implants each year.
Nevertheless, such cochlear implants and brain-stem implants cannot restore hearing correctly for various reasons.
Firstly, those implants all rely on the concept of electrically stimulating neural tissue. Unfortunately, such stimulation is always associated with electrical diffusion phenomena in tissue and in particular in sensory tissue. Thus, any electrical stimulation of neural tissue ends up addressing a plurality of neural circuits without sufficient mastery over specificity.
Furthermore, the electrical stimulation devices that have been developed for cochlear implants or for brain-stem implants all have electrodes, and are thus highly invasive for the patients, thereby damaging or even completely destroying the weak natural residual auditory function.
Furthermore, electrical stimulation systems require implantation in or in contact with neural tissue, which also involves a risk of electrodes being wrongly or poorly positioned.
Finally, the electrical stimulation electrodes of cochlear implants and of brain-stem implants cannot comply accurately with the anatomic and functional organization of the neurons (somatotopy) insofar as the implantation zones are very dense in neurons, once more making it likely that several neural circuits will be activated simultaneously.
Consequently, patients having such implants can unfortunately perceive only very few different sounds, which does not enable them to follow a normal oral conversation at a usual sound level.
In order to solve some of those technical problems, research has been directed to developing electrical “grids” for positioning in direct contact with the surface of the temporal lobe of the brain. Nevertheless, many of the above-mentioned problems remain.
During electrical stimulation of such surface cortical grids, it is not possible to preserve somatotopy and diffusion of the peripheral cortex of the primary auditory cortex. Furthermore, since the primary auditory cortex is located more deeply than the surface of the temporal lobe on which the grid is applied, the resulting electrical excitation does not take place in the appropriate gyrus.
Other research teams have attempted to solve these problems by inserting brain electrodes. Nevertheless, that technique is particularly invasive and the movements of the electrodes, and of the grids, are incompatible with the physiological pulsatility of the brain inside the skull.
U.S. Pat. No. 4,982,434 discloses a system for treating deafness that is for fastening to a patient's skull. That system includes a microphone suitable for picking up ambient sounds. The microphone is connected via a converter to a transducer for applying vibrations to the skull of the patient. The vibrations applied to the skull are transmitted to the inner ear saccule that activates the cochlear nerve. Such a treatment system seeking to stimulate the auditory cortex indirectly via the inner ear is not suitable for correctly restoring hearing for the reasons mentioned above.
In the field of visual acuity, blindness and visual difficulties nowadays constitute a major medical problem insofar as these pathologies have considerable repercussions on the social lives of the patients concerned, who represent about 0.5% of the population. There are many potential causes for blindness: malformation; infection (toxoplasmosis); trauma; tumors (brain tumors); degenerative (age-related macular degeneration (ARMD)); vascular (diabetes, stroke). These pathologies may occur in the eyeball, and also anywhere along the nerve transmission path (optical nerve, chiasma, optic tracts, external geniculate bodies, optic radiation, visual cortex).
In the event of severe visual difficulties and blindnesses that cannot be improved medically or surgically, very artificial solutions for restoring vision have already been envisaged:
If the pathology involves the eyeball itself by damage to the cornea or the lens, the problem is not neurological but optical. Thus, artificial lenses are implanted to take the place of the cornea or of the natural lens.
If the pathology concerns neurological optical pathways, then there is at present no satisfactory system. Nevertheless, a large amount of research work has been carried out. It concerns either restoring the visual pathways by grafts or else developing artificial systems with detection by camera. An image picked up by a camera is processed and then used for activating defective natural visual pathways via an electrical interface. Those thus constitute devices for electrically stimulating neural tissue.
Electrical stimulation can be performed on the ganglion cells of the retina by installing plates of 16 to 40 electrodes.
The electrical stimulation may be performed directly on neural tissue (external geniculate ganglions, cerebral visual cortex) by putting plates into place having up to 242 electrodes, either on the surface of the visual cortex or implanted within the cortex.
At present, the most advanced appliances provide only rudimentary images in black and white that are still not sufficient for finding one's location in unknown surroundings. Implanting 242 electrodes in a brain can obtain tunnel vision only, being equivalent to rendering a photograph on a matrix of 15 pixels by 16 pixels. Several problems persist that explain this poor effectiveness:
The primary visual cortex is mainly deployed on the medial face of the brain, i.e. an internal face, where it is therefore difficult to put a deep electrode plate into position.
Plates with electrodes that penetrate into the brain provide the best presently-available visual rendering, but they are very traumatic for the cortex, and the quality of the connection deteriorates over time because of the appearance of insulating layers and because of the physiological pulsatile movements of the brain inside the skull.
Electrical stimulation is always associated with electrical diffusion phenomena within tissue, and in particular within sensory neural tissue. Thus, any electrical stimulation of neural tissue ends up addressing a plurality of neural circuits without specificity being sufficiently well controlled. It has been found that electrode density is presently limited not by manufacturing limits but by the effects of current and short circuits between electrodes that are too close together.
Thus, electrical stimulation electrodes cannot comply closely with the anatomical and functional organization of neurons (somatotopy) insofar as the implantation zones are very dense in neurons, once more giving rise to several neural circuits being activated simultaneously. Thus, such a treatment system seeking to stimulate the visual cortex indirectly is not suitable for correctly restoring vision for the reasons mentioned above.
In an entirely different field, it has nevertheless recently been shown that the use of pulsed focused ultrasounds can stimulate intact motor zones of the brain in non-invasive manner. Modulating neurons with ultrasound thus appears to be a technique that is favorable for designing non-invasive brain/machine interfaces for stimulating the brain.
By way of example, it has been shown in rats that ultrasound stimulation of the motor cortex leads to sufficient neural activity to trigger motor behavior of the subject (Y. Tufail, A. Matyushov, N. Baldwin, M L Tauchmann, J. Georges, A. Yoshihiro, S. I. Tillery, W J Tyler, “Transcranial pulsed ultrasound stimulates intact brain circuits” Neuron, Jun. 10, 2010; 66(5): pp. 681-694).
Such coupling of ultrasound techniques with neural physiology was mentioned for the first time in 2006 by Rvachev (M. M. Rvachev, “Alternative model of propagation of spikes along neurons”, Physics 2006).